This invention relates to apparatus and methods for detecting and locating faults or imperfections, such as cracks and fractures, in steel tapes and, particularly, perforated drive tapes, such as those used in elevator systems, generally known as selector tapes.
Selector tapes are employed in many elevator installations. The tape is constructed of ductile steel and is connected to the car as a continuous belt: one of its ends is attached to the top of the car and its other end is attached to the bottom. As the car ascends and decends in the hoistway, the tape drives a mechanical encoder which provides relative positional information as the car's location and movement in the shaft. The encoder is generally known as the selector. The selector tape usually contains perforations to mate with the teeth on a sprocket wheel that is rotated to drive the selector as the car ascends and decends. The perforations quite obviously provide reliable, positive mechanical connection to the selector to achieve precise, repeatable positional information. The tape is rather thin in order to give it a necessary level of ductility for it to loop smoothly around the sprocket.
Therefore, as the car moves within the elevator shaft, the selector tape is repeatedly bent and unbent or straightened as it moves around the sprocket to drive the selector. As a result, after extended service the tape can fatigue, which usually appears as small hairline cracks or fractures at first. If these are allowed to spread across the tape, a complete breakage can occur.
In very high buildings the selector tape can be several hundreds of feet long and, therefore, it is particularly prone to develop oscillations and thus continuously wiggle or sway in the elevator shaft. This can further accelerate tape fatigue. In addition, the tape's proneness to sway in tall buildings is increased by the tendency for tall buildings to sway in the wind. In fact, sometimes the swaying becomes so violent, as a result of car movement and building movement, that the tape strikes the cables connected to the car. This is a rare occurrence, however. In tall buildings tape fatigue therefore can develop more rapidly than in smaller buildings, where the tape is naturally much shorter.
If allowed to go uncorrected, tape fatigue can lead to complete tape breakage, in some cases, in which instance the car will be without any positive positional information, simply because the encoder or selector would not be driven. In the typical elevator installation, however, there are backup safety devices which prevent further car movement, except possibly to a near floor, in the event of complete breakage. Clearly, however, it is completely undesirable to allow tape fatigue to advance, unnoticed, to the point where complete breakage can occur. The only way to avoid that is to detect or diagnose fatigue in its early stages so that the tape can be repaired or replaced. Tape repair generally involves splicing in a new section into the tape. The detection procedure most commonly used consists simply of running the elevator up and down while a service technician holds his fingernail on the tape in order to detect the rough edge of a crack. Clearly this is a less than desirable procedure, particularly since it is slow and uneconomical.
Wire cables and rope are also subject to fatigue, in the form of broken wires, for much the same reasons. A prior art technique for detecting broken wires is to induce magnetic flux in the rope and sense the flux radiating from the rope as a result of the broken wires. This involves moving the rope so that the flux intersects a magnetic pickup device which thereupon produces a signal indicating the presence of a broken wire.
Although this technique is acceptable for steel ropes, it has no utility with selector tapes simply because this technique cannot distinguish between perforations and cracks. In other words, the tape's perforations will appear to be cracks. Hence, this technique evidences the distinct need for a crack detection technique specifically suited for metal tapes and the like which contain spaced apart perforations.